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Several recent papers have argued for warm, semi-detached, molecular layerssurrounding red giant and supergiant stars, a concept known as a MOLsphere.Spectroscopic and interferometric analyses have often corroborated this generalpicture. Here, we present high-resolution spectroscopic data of pure rotationallines of water vapor at 12 microns for the supergiant mu Cephei. This star hasoften been used to test the concept of molecular layers around supergiants.Given the prediction of an isothermal, optically thick water-vapor layer inLocal Thermodynamic Equilibrium around the star (MOLsphere), we expected the 12micron lines to be in emission or at least in absorption but filled in byemission from the molecular layer around the star. Our data, however, show thecontrary; we find definite absorption. Thus, our data do not easily fit intothe suggested isothermal MOLsphere scenario. The 12 micron lines, therefore,put new, strong constraints on the MOLsphere concept and on the nature of waterseen in signatures across the spectra of early M supergiants. We also find thatthe absorption is even stronger than that calculated from a standard,spherically symmetric model photosphere without any surrounding layers. A coolmodel photosphere, representing cool outer layers is, however, able toreproduce the lines, but this model does not account for water vapor emissionat 6 microns. Thus, a unified model for water vapor on mu Cephei appears to belacking. It does seem necessary to model the underlying photospheres of thesesupergiants in their whole complexity. The strong water vapor lines clearlyreveal inadequacies of classical model atmospheres.Comment: Accepted for publication in the Astrophysical Journa

Water Vapor on Supergiants: The 12 μm TEXES Spectra of μ Cephei